Strange strobing effect when PC & projector are connected to different power sources

[MrFlibble]

In a particular classroom if I plug the projector into the mains power outlet in the ceiling, and the PC into a mains power outlet in the wall, there is a strange strobing effect scrolling vertically down the projected image.

 

The image is absolutely fine with no problems whatsoever if the projector is, like the PC, also plugged into an adjacent socket in the wall.

 

It's not an issue with the projector (I tried two), or the PC (I tried two, of different models), or the VGA cable (we are using a splitter but I got a brand new cable and used this to connect the PC directly into the projector).

 

The problem is more noticeable when using a splitter, and even more so when using a long VGA cable (both of which would be required in normal use), but even using a short VGA cable with no splitter it's still noticeable.

 

I do not believe this is an 'IT problem' as such, but some sort of power problem. However our site manager (who was previously a qualified electrician) is at a loss as to why this could be occurring. I believe the socket in the ceiling probably goes to a different distribution board to the sockets in the wall. Could it be something to do with the 'phase' of the electricity or something?

 

Has anyone experienced a problem like this before, and if so what was the solution?

[ZeroHour]

Are they on the same power ring?

I have seen a bizarre problem in a lab where CRT monitor on pcs at the end of the ring looked like they had magnets near the displays. If you moved crt's to a different socket it wouldnt happen, in the end we had to get a second ring installed and that fixed the issue but it was a bizarre problem.

Dont know if its relevant though with projectors though.